Graphene-based electrode for a supercapacitor

ABSTRACT

A supercapacitor electrode mechanism comprising an electrically conductive, porous substrate, having one or more metallic oxides deposited on a first surface and a chemically reduced graphene oxide deposited on a second surface, to thereby provide an electrical double layer associated with the substrate. The substrate may be carbon paper or a similar substance. The layers of the supercapacitor are optionally rolled into an approximately cylindrical structure.

ORIGIN OF THE INVENTION

The invention described herein was made in the performance of work undera NASA contract and by an employee of the United States Government andis subject to the provisions of Public Law 96-517 (35 U.S.C. §202) andmay be manufactured and used by or for the Government for governmentalpurposes without the payment of any royalties thereon or therefore. Inaccordance with 35 U.S.C. §202, the contractor elected not to retaintitle.

FIELD OF THE INVENTION

This invention relates to nanofabrication of an electrode suitable foruse in a supercapacitor.

BACKGROUND OF THE INVENTION

A supercapacitor requires use of a substance that has a relatively highpower density, which can be achieved with some materials, andsimultaneously a relatively high energy density. Achievement of both ofthese conditions with a single material has not been possible in theprior art.

SUMMARY OF THE INVENTION

The invention meets these needs by providing a procedure fornanofabrication of an electrode mechanism, for use in a supercapacitor(SC) that has relatively high power density (100-200 KW/Kgm or higher)and simultaneously has relatively energy density (20 KW-hr/KGM orhigher). The procedure includes the steps of: (1) providing a porous,electrically conductive substrate that has first and second, spacedapart surfaces; (2) depositing one or more selected metal oxides,including at least one of MnO₂ and Co₃O₄, on the first substratesurface; and (3) depositing an assembly of chemically reduced grapheneoxide (rGO) on the second substrate surface, to thereby provide anelectrical double layer associated with the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart of a procedure for fabricating an electrodemechanism according to an embodiment of the invention.

FIG. 2 is a schematic view of a layered embodiment of the invention.

FIG. 3 illustrates a perspective end view of a cylindrical embodiment ofthe invention.

DESCRIPTION OF THE INVENTION

FIG. 1 is a flow chart of a procedure for fabricating an electrodemechanism according to an embodiment of the invention. In step 11, aporous, electrically conductive substrate is provided that has spacedapart, first and second, surfaces. The substrate can be a metal, such asstainless steel or thin metal foil, a doped semiconductor, such as Sidoped at 10¹⁹ cm⁻³ or higher, carbon paper, such as Toray paper or Buckypaper, an electrically conducting polymer, or a cellulose substance(optionally including photocopy paper) impregnated with a combination ofcarbon nanotube (CNT) ink and sodium dodecylbenzesesulfonate, the carbonpaper having a thickness in a range 500-2000 nm, or another conductivesubstance.

In step 12, one or more selected metal oxides, including at least one ofMnO₂ and Co₃O₄, is deposited on the first substrate surface. The metaloxide has an associated specific capacity of around 300 Farads/gm.

In a third step 13, an assembly of flakes or chips of reduced grapheneoxide (rGO) is deposited on the second substrate surface, withindividual flake thicknesses in a range 1-15 μm, depending upon theelectrolyte. Deposition of the rGO can, for example, be achieved bydeposit of graphene oxide (GO) on the substrate second surface, followedby electrophoretic deposition (EPD), such as cathodic deposition usingan electrolyte such as a room temperature ionic liquid (RTIL).

An RTIL is a room temperature, liquid, solvent-free electrolyte.Examples of RTILs are 1-butyl-1-methylpyrrolldiniumbis(trifluoromethylsulfonyl)imide, Methyltrioctylammoniumbis(trifluoromethylsulfonyl)imide, ammonium ions, and methyl imidazole.

The reducing agent may be hydrazine, sodium borohydride, or a similarsubstance. The rGO helps provide an electrical double layer associatedwith the substrate.

FIG. 2 illustrates a three-layer, substantially planar embodiment 20 tof the invention provided by the procedure discussed in FIG. 1,comprising a metallic oxide layer 21, a substrate 22 and an rGO layer23, with associated electrical terminals 24 and 25. The electrolyte maybe an RTIL or a combination of RTIL and a solid polymer (SP) thatmanifests at least some piezoelectric behavior.

This fabrication procedure also provides a three-layer (more generally,a multi-layer) cylindrical embodiment 30 in FIG. 3 for storage ortransfer of an electrical charge with relatively high power density andenergy density. The energy density and power density can be increasedsubstantially by rolling this three-layer system into a3N-layercylindrical system 30, as indicated in an end view in FIG. 3,with N>>1. In FIG. 3, the geometric structure is illustrated with Napproximately equal to 2. The first layer 31 contains metallic oxides,the second layer 32 is a thicker substrate layer, and the third layer 33includes rGO and a selected electrolyte. The substrate may have athickness as small as 1-5 μm and as large as 150 μm depending upon thesubstrate material and the maximum permissible curvature of thegeometric structure illustrated in FIG. 3. For a given substratematerial, the smaller the substrate thickness, the greater the geometricstructure that can be supported without substrate cracking or crazing.The layers illustrated in FIG. 3 are preferably rolled into a structureresembling a cylinder.

In an embodiment, illustrated in FIG. 2 or FIG. 3, the (solid orgel-like) electrolyte comprises a combination of an RTIL and a solidpolymer SP, such as polyvinylidene fluoride or difluoride, preferablyβ-phase.

Ideally, the system should operate at temperatures in a range −40°C.<T<100° C., should operate at voltages in a range −3 to +7 Volts, andshould rely upon non-toxic materials for fabrication. The system shouldbe mechanically flexible and have a relatively small form factor.

It will be appreciated to those skilled in the art that the precedingexamples and embodiment are exemplary and not limiting to the scope ofthe present invention. It is intended that all permutations,enhancements, equivalents, combinations, and improvements thereto thatare apparent to those skilled in the art upon a reading of thespecification and a study of the drawings are included within the truespirit and scope of the present invention.

What is claimed is:
 1. A method for fabricating a supercapacitor, themethod comprising: providing an electrically conductive substrate,having spaced apart first and second surfaces; providing a layer ofmetal oxides, including at least one of MnO₂ and Co₃O₄, contiguous tothe first substrate surface; and providing a layer of chemically reducedgraphene oxide (rGO) contiguous to the second substrate surface, tothereby provide an electrical double layer associated with thesubstrate.
 2. The method of claim 1, further comprising providing, assaid conductive substrate, a sheet of carbon paper comprising acellulose substance impregnated with a combination of carbon nanotubeink and sodium dodecylbenzenesulfonate.
 3. The method of claim 2,wherein said carbon paper has an associated capacitance of at leastabout 200 Farads/gm.
 4. The method of claim 1, further comprisingproviding, as said conductive substrate, at least one of a layer ofstainless steel, a thin metal foil and a semiconductor material with adopant density of at least 10¹⁹ cm⁻³.
 5. The method of claim 1, furthercomprising providing said rGO layer on said second substrate surface bya process comprising depositing a layer of graphene oxide on said secondsubstrate surface, using electrophoretic deposition comprising providingat least one room temperature ionic liquid (RTIL) as an electrolyte forsaid rGO.
 6. The method of claim 1, further comprising providing saidrGO layer on said second substrate surface by a process comprisingdepositing a layer of graphene oxide on said second substrate surface,using electrophoretic deposition comprising at least one roomtemperature ionic liquid (RTIL) as an electrolyte and at least one solidpolymer that manifests piezoelectrical behavior.
 7. The method of claim5, further comprising choosing said room temperature ionic liquid tocomprise at least one of methyl imidazole, ammonium ions,1-butyl-1-methylpyrrolldinium bis(trifluoromethylsulfonylimide, andmethyltrioctylammonium bis(trifluoromethylsulfonyl)imide.
 8. The methodof claim 5, further comprising forming said electrical double layer atan interface between said reduced graphene oxide layer and saidelectrolyte.
 9. The method of claim 1, further comprising providing saidreduced graphene oxide using a substance comprising at least one ofhydrazine and sodium borohydride as a reducing agent.
 10. The method ofclaim 1, further comprising providing a distribution of said metaloxides in a first layer that serves as a first electrical terminal,providing said substrate in a second layer, providing a distribution ofsaid chemically reduced graphene oxide in a third layer that serves as asecond electrical terminal, and forming the first, second and thirdlayers into a cylindrical structure.